This invention relates to an improved polymerization process. More particularly, this invention relates to an improvement in processes which comprise an initial batch polymerization reaction and a subsequent semi-batch or continuous reaction, with both reactions carried out under an anionic polymerization mechanism.
A variety of valuable synthetic thermoplastic rubber products are conventionally produced by the anionic polymerization of monomers such as conjugated dienes, alkenyl aromatic compounds and activated mono-olefins. Anionic polymerization is characterized by the initiation and propagation of what are commonly termed living polymer chains. When the chains are initiated at substantially the same time and grow at substantially the same rate, the polymerization reaction yields a product having a very narrow molecular weight distribution. The molecular weight of each individual polymer chain is relatively close to the average molecular weight of the overall product, i.e., the total weight of the monomer(s) consumed divided by the moles of initiator used. Uniformity of the polymer molecules in the products of anionic polymerization is an important factor from the standpoint of many of the desirable physical properties of the materials.
In the practice of anionic polymerization processes, it is often the case, however, that a portion of the living polymeric chains are terminated before they reach their desired, or "target", molecular weight. Moreover, because these terminated chains do not reach their target molecular weight, the polymerization mixture contains an excess of monomer which was intended for reaction with the terminated chains. This excess monomer adds onto the remaining live chains to form chains having a molecular weight higher than that originally targeted. The net effect of chain termination is then a broadening of the molecular weight distribution of the product both below and above the target.
It is recognized in the art that the principal mechanisms for chain termination in anionic polymerization are (i) a reaction between the active anionic polymerization site of the living chain and one or more impurities in the polymerization system, and (ii) thermal die-out of the active site at the higher polymerization temperatures, e.g., temperatures greater than about 50.degree. F. Termination via the impurities reaction can usually be avoided or at least controlled within acceptable limits by maintaining the purity of the monomer, polymerization initiator, and diluent in the polymerization mixture. Termination via thermal die-out, which is thought to involve the elimination of the living anionic site from the chain through a reaction forming a hydride, is in many cases more difficult to bring under control.
It is the principal object of this invention to minimize the incidence of chain termination reactions in certain anionic polymerization processes, particularly termination which can be attributed to a thermal die-out mechanism.
The invention is particularly intended for application to anionic polymerization processes which are initiated in a batch mode, and then subsequently continued in a continuous (or semi-continuous) mode. Such processes are well known in the art and are in common commercial practice. For the batch mode process step, predetermined amounts of (i) one or more monomers suitable for anionic polymerization and (ii) an anionic polymerization initiator, are contacted to form and propagate the living polymeric chain. This step is strictly a batch operation in the sense that all monomer and all initiator are brought into contact at the same time; no further addition of monomer or initiator takes place during this step, nor is any removal made from the contact mixture of polymerization product. The relative proportions of monomer and initiator used in this step are such that the chains do not reach their target molecular weight in the batch step alone, and polymerization is subsequently continued (with the same and/or different monomer) in a second step which is operated in a continuous or semi-continuous mode. This second reaction step is termed continuous in the sense that monomer is added (continuously, or also suitably intermittently) to the reaction mixture as the polymerization proceeds. However, no removal of product is made until the chains reach the target molecular weight.